As semiconductor manufacturing advances from very large scale integration (VLSI) to ultra-large scale integration (ULSI), the devices on semiconductor wafers shrink to sub-micron dimension and the circuit density increases to several million transistors per die. In order to accomplish this high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines in the service geometry such as corners and edges, of various features.
The requirement of small feature sizes with close spacing between adjacent features requires high resolution photolithographic processes. In general, photolithography utilizes a beam of light, such as ultra violet (UV) waves, to transfer a pattern from a photolithographic mask to a photo resist coating through an imaging lens. The mask includes opaque and transparent regions such that the shapes match those openings in the resist coating in the desired or predetermined pattern.
Early prior art masks, and referring now to FIG. 1A-1D, were made by coating a quartz blank 10 with a chrome 12 or other light blocking material layer upon which was laid a coating of photoresist material 16. An electron beam 15 used to write the desired pattern on the photoresist coating 16 after which the photoresist is developed. Next, and referring to FIG. 1C, a chrome etch is performed on the exposed regions, after which the resist is stripped. As shown in FIG. 1D, a reticle having transparent openings 14 and opaque area 12 has been formed on the quartz blank 10. Unfortunately, reticles thus formed are limited in resolution capability due to the effects of diffraction.
One technique currently being investigated for improving the resolution of the photolithographic process is known as phase shift lithography. With phase shift lithography, the interference of light rays is used to overcome diffraction and improve the resolution and depth of optical images projected on to a target. In phase shift lithography, the phase of an exposure light at the object is controlled such that adjacent bright areas are formed preferably 180.degree. out of phase with one another. Dark regions are thus produced between the bright areas by disruptive interference even when diffraction would otherwise cause these areas to be lit. This technique improves total resolution at the object, providing a line/space resolution 50% to 70% greater than that possible using conventional transmission means.
An early patent in this field, U.S. Pat. No. 4,360,586 to Flanders et al, is directed to exposing periodic optical features on an object surface. The features were characterized by a spacial period "p". According to the invention, a source of radiant energy of wavelength illuminates a surface to be exposed through a mask having a spatial period separated from the surface by a distance approximately S.sub.n =P.sup.2 /.lambda., where n is an integer greater than 1.
With respect to semiconductor fabrication numerous laboratory techniques have been proposed to employ phase shifting in the photo patterning of semiconductor wafers. Most of the work in this area has centered around either "Alternating Phase Shifting", "Subresolution Phase Shifting", or "Rim Phase Shifting" experiments. In general, in each of these techniques a phase shift mask or reticle is constructed in repetitive patterns in three distinct layers of material. An opaque layer on the mask provides areas that allow no light transmission therethrough, a transparent layer provides areas which allow close to 100 percent of light to pass through and a phase shift layer provides areas which allow close to 100 percent of light to pass through, but phase shifted 180.degree. from the light passing through the transparent areas. The transparent areas and phase shifting areas are situated so that light rays defracted through each area is cancelled out in a darkened area there between. This creates the pattern of dark and bright areas which can be used to clearly delineate features of a pattern on a photopattern wafer.
"Alternating Phase Shifting" is a spatial frequency reduction concept similar to the method disclosed in the Flanders et al patent. It is characterized by a pattern of features alternately covered by a phase shifting layer. "Subresolution Phase Shifting" promotes edge intensity cut off by placing a subresolution feature adjacent to a primary image and covering it with a phase shifting layer. "Rim Phase Shifting" overhangs a phase shifter over a chrome mask pattern.
In general, these phase shifting techniques have not been adapted to large scale semiconductor manufacturing processes. One problem with applying phase shifting lithography into practical use, in manufacturing semiconductors, is the difficulty in reticle mask making, inspection, and repair. The process must be compatible with the manufacturing conditions, (ie. inexpensive, repetitive, clean) and prior art laboratory techniques have not heretofore met these criterion.
A representative state of the art semiconductor laboratory process for making a phase shift mask or reticle is the process generally disclosed in the Flanders et al patent. This process is shown in FIGS. 2A-2D and can be termed a "lift-off or additive process".
The "lift-off process" may be used to fabricate a reticle or hard copy of an individual drawing for a semiconductor circuit layout. The reticle may then be used directly as a mask in the photopatterning process or may be used to produce a photomask. As an example, the reticle may be used to pattern a wafer surface in a stepped pattern transfer. DRAMs and SRAMs because of their repetitive nature are adapted to manufacture in this manner.
Still referring now to FIG. 1D, with the lift-off process, a transparent quartz substrate 10 has a film of an opaque material 12 such as chromium (CR) pattern thereon. The chromium 12 may be deposited and patterned onto the substrate 10 by a conventional process such as electron beam lithography. In the example of FIG. 1D, the pattern is a periodic arrangement of chrome 12 and spaces 14 patterned on the quartz substrate 10.
Referring now to FIG. 2A, a thin film layer of phase shifter material such as (SiO.sub.2) is deposited over the pattern chrome 12 and spaces 14. The phase shift material is selected from those with suitable optical properties and is deposited with a thickness equivalent to a half wavelength (.lambda./2) at the exposure source of interest. Thus, the phase shift ultimately obtained as a function of the thickness "t" and refractive index of the phase shifter material, which are preferably selected to provide a 180.degree. phase shift.
Next, the phase shift material is coated with a resist 20 sensitive to electron beam exposure. The desired phase shift pattern is then written onto the resist 20 and the resist pattern thus obtained is developed and removed. Next, the phase shift film 18 is etched in the regions where the resist was removed. Following this step, the resist is then stripped, leaving a phase shifter 18 in every other opening 14 between the chrome 12. This provides an alternating phase shifting pattern as previously explained.
Problems with this "lift-off process" are that it is defect prone, inconsistent, a less manufacturable procedure and, thus, not as suitable for large scale manufacturing. Deposition of the phase shifter material is prone to particulate contamination. These contaminants may cause subsequent patterning errors of the finished wafer. Another problem with the "lift-off process" is that it is difficult to accurately control the thickness of phase shifter 18 uniformally across the quartz plate 10. A still further problem is the lack of an etch stop between the phase shifter film and the quartz substrate (particularly when SiO.sub.2 is used for both). A uniform timed etch accurate across the whole of the quartz plate is difficult to achieve.
In the finished reticle, each phase shifter 18 preferably has a thickness "t" (FIG. 2C) that produces a 180.degree. phase shift for light passing there through. This optimal thickness can be determined by the formula: ##EQU1## where t=thickness of phase shift material
i=an odd integer PA1 .lambda.=equals wavelength of exposure light PA1 n=refractive index of phase shifter material of the exposure wavelength
Any deviation from this optimal thickness adversely effects the phase shift ultimately obtained. An "edge effect" may occur, for example, at the edge of a phase shifter 18 deposited over the edge of a chrome light blocker 12. This may be due to the conformal deposition of the phase shifter material 18 over the chrome light blocker 12 which causes the phase shifter 18 to be slightly thicker at the edges.
Another prior art method for forming a phase shift reticle is shown in FIGS. 3A-3C. As before, a pattern of opaque light blockers 12 such as chromium (CR) is formed on a quartz substrate 10. As shown in FIG. 3A, photoresist 20 is then pattern on the chrome light blockers 12 in every other opening 14. As shown in FIG. 3B, openings 14 not covered with photoresist are then etched to form phase shifter openings 22 etched back to a depth "d" into the substrate of the quartz substrate 10. Light passing through an etched back phase shifter opening 22 is phase shifted relative to light passing through an adjacent transmission opening 14 which must travel through the full thickness of the quartz substrate 10.
This prior art reticle fabrication method is also subject to several disadvantages. First, the etch depth "d" into the quartz 10 requires a highly controlled uniform etch over a relatively large (example--5") reticle surface. Since it is necessary to phase shift within a margin of .+-.10.degree. or less, this translates to an etch tolerance of about 5%. This is difficult to achieve under manufacturing conditions.
Further, the prior art etch back technique shown in FIGS. 3A-3C causes a finite loss in light transmission due to the roughness of the exterior surface of the etch back phase shifter openings 22. Another disadvantage of this etch back technique is the difficult requirement of having to accurately pattern photoresist 20 over the topology of the opaque light blockers 12.
The process of the present invention is directed to a novel process that overcomes these prior art limitations. The process of the invention provides a clean, repetitive technique for forming accurate phase shifting reticles suitable for large scale semiconductor manufacturing. Moreover, the process of the invention can be used to provide phase shifters having a smooth and optically flawless surface.